In quantum electrodynamics of two space dimensions, a quantum Hall effect occurs in the absence of any magnetic field. We give a simple and transparent explanation. In solid state physics, the Hall conductivity for non-degenerate ground state is expected to be given by an integer, the Chern number. In our field-free situation, however, the conductivity is ±1/2 in natural units. We fit this half-integral result into the topological setting and give a geometric explanation reconciling the points of view of quantum field theory (QFT) and solid state physics. For quasiperiodic boundary conditions, we calculate the finite size correction to the Hall conductivity. Applications to graphene and similar materials are discussed.

Dimer models are 2-dimensional combinatorial systems that have been shown to encode the gauge groups, matter content and tree-level superpotential of the world-volume quiver gauge theories obtained by placing D3-branes at the tip of a singular toric Calabi–Yau cone. In particular the dimer graph is dual to the quiver graph. However, the string theoretic explanation of this was unclear. In this paper we use mirror symmetry to shed light on this: the dimer models live on a T² subspace of the T³ fibre that is involved in mirror symmetry and is wrapped by D6-branes. These D6-branes are mirror to the D3-branes at the singular point, and geometrically encode the same quiver theory on their world-volume.

In this paper, we study the perturbative aspects of the half-twisted variant of Witten’s topological A-model on a complex orbifold X/G, where G is an isometry group of X. The objective is to furnish a purely physical interpretation of the mathematical theory of the Chiral de Rham complex on orbifolds recently constructed by Frenkel and Szczesny in Chiral de Rham complex and orbifolds, Preprint, arXiv: math.AG/ 0307181. In turn, one can obtain a novel understanding of the holomorphic (twisted) N = 2 superconformal structure underlying the untwisted and twisted sectors of the quantum sigma model, purely in terms of an obstruction (or a lack thereof) to a global definition of the relevant physical operators which correspond to G-invariant sections of the sheaf of Chiral de Rham complex on X. Explicit examples are provided to help illustrate this connection, and comparisons with their non-orbifold counterparts are also made in an aim to better understand the action of the G-orbifolding on the original half-twisted sigma model on X.

The noncommutative self-dual φ³ model in six dimensions is quantized and essentially solved, by mapping it to the Kontsevich model. The model is shown to be renormalizable and asymptotically free, and solvable genus by genus. It requires both wavefunction and coupling constant renormalization. The exact (“all-order”) renormalization of the bare parameters is determined explicitly, which turns out to depend on the genus 0 sector only. The running coupling constant is also computed exactly, which decreases more rapidly than predicted by the 1-loop beta-function. A phase transition to an unstable phase is found.

We first review the description of flag manifolds in terms of Plücker coordinates and coherent states. Using this description, we construct fuzzy versions of the algebra of functions on these spaces in both operatorial and star product language. Our main focus is here on flag manifolds appearing in the double fibration underlying the most common twistor correspondences. After extending the Plücker description to certain supersymmetric cases, we also obtain the appropriate deformed algebra of functions on a number of fuzzy flag supermanifolds. In particular, fuzzy versions of Calabi–Yau supermanifolds are found.